Currently there are various types of artificial heart valves which have been proposed for clinical use. They include: (1) those made of a rigid metal frame with a central occluder which functions as a check valve with each beat of the heart, (2) tissue valves made from an animal's (pig) heart valve that is stretched and sewn over a rigid metal framework to provide central flow of blood, and (3) tri-leaflet valves of limited longevity and reliability. Such valves have never been clinically acceptable.
Central occluder valves consist, typically, of "ball-in-cage" or low-profile "disc-in-cage" designs utilizing plastic, silicone rubber, carbon or metal occluders. The rubber occluders suffer from two major shortcomings: (a) they are subject to "lipid" adsorption from the blood stream with concomitant changes in dimensions and physical integrity; splitting, tearing, and clotting result (see:
Bonnabeau, R. C. Jr., and Lillehei, C. W.: "Mechanical ball failure in Starr-Edwards Prosthetic Valves", J. Thorac. Cardiovas. Surg. 56: 258, 1968;
Hylen, J. C.: "Durability of Prosthetic Heart Valves", Am. Heart J. 81: 299, 1971;
Hylen, J. C., Hodam, R. P. and Kloster, F. E.: "Changes in the Durability of Silicone Rubber in Ball-Valve Prostheses", Annals Thorac. Surg. 13: 324, 1972;
Aston, S. J. and Mulder, D. G.: "Cardiac Valve Replacement, A Seven-Year Follow-up", J. Thorac. Cardiovas. Surg. 61: 547, 1971;
Starr, A., Pierie, W. R., Raible, D. A., Edwards, M. L., Siposs, G. C., and Hancock, W. D.: "Cardiac Valve Replacement, Experience With the Durability of Silicone Rubber", Suppl. I to Circulation, 33, 34, April 1966) and
(b) they possess a low order of wear resistance (see Boretos, J. W., Detmer, D. E. and Donachy, J. H. "Segmented Polyurethane: A polyether Polymer, II, Two Years Experience", J. Biomed. Mater. Res. 5: 373, 1971).
Clinical evidence with the "low profile" design exemplifies this characteristic, (Detmer, D. E.; McIntosh, C. L.; Boretos, J. W.; Braunwald, N. S.: "Polypropylene Poppets for Low-Profile Prosthetic Heart Valve", Annals Thorac. Surg. 13: 122, 1972).
The metal and carbon occluders are: (a) abrasive to cloth covered struts causing fragmentation of the cloth with ensuing emboli, (Detmer, D. E. and Braunwald, N. S.: "The Metal Poppet and the Rigid Prosthetic Valve", J. Thorac. Cardiovas. Surg. 61: 175 1971;
Ablaza, S. G. G., Blanco, G., Javan, M. B., Maranhao, V. and Goldberg, H. "Cloth Cover Wear of the Struts of the Starr-Edwards Aortic Valve Prosthesis", J. Thorac, Cardiovas. Surg. 61: 316, l971;
Thomas, C. S., Killen, D. A., Alford, W. C., Burrus, G. R. and Stoney, W. S.: "Cloth Disruption in the Starr-Edwards Composite Mitral Valve Prosthesis", Annals Thorac. Surg. 15: 434, 1973).
Cloth covering has been shown to minimize thrombus formation on metal frames and is generally considered necessary for these designs, (Detmer, D. E. and Braunwald, N. S. "The Metal Poppet and the Rigid Prosthetic Valve", J. Thorac. Cardiovas. Surg. 61: 175, 1971;
Ablaza, S. G. G., Blanco, G., Javan, B. B., Maranhao, V. and Goldberg, H. "Cloth Cover Wear of the Struts of the Starr-Edwards Aortic Valve Prosthesis", J. Thorac. Cardiovas. Surg. 61: 316, 1971 ).
However, paravalvular leaks caused by cloth wear on the valve seats are responsible for high blood hemolysis, (Thomas, C. S., Killen, D. A., Alford, W. C., Burrus, G. R. and Stoney, W.: "Cloth Disruption in the Starr-Edwards Composite Mitral Valve Prosthesis", Annals Thorac. Surg. 15: 434, l973).
The metal and carbon occulders are also: (b) abrasive to bare metal valve surfaces, especially for metal to metal contact, and (c) hard and they accordingly generate a "clicking" sound with each cycle. This noise and its associated anticipation is highly distressing to patients. In addition, in metal and carbon occluders (d) the central flow of blood is blocked by the presence of the occluder, reducing the potential volume output, increasing resistance to flow, and causing turbulance of flow which is believed to add to the incidence of thrombo embolism.
Tissue valves are generally made by sewing excised pig valves over a rigid framework. These have the following disadvantages: (a) Tissue valves individually variable due to the fact that they are extracted from a living animal and subject to extremes of physical and physiological differences which have occured during growth of the animal (b) There is no absolute way of testing individual valves for strength and durability prior to use. (c) Fixation techniques must be used to preserve tissue valves after extraction and during fabrication and storage and bacterial invasion has been difficult to control. Absolute sterility is difficult to assure, and patients occasionally become physically distressed due to bacterial endocarditis following surgical placement of a tissue valve. (d) Long-term implants have shown evidence of calcium deposits generated on the surface of the leaflets in some cases rendering them stiff and non-functional. (e) Tissue valves are not suitable for use in artificial heart assist devices where assembly, storage, and sterilization of the device must be done well in advance of surgery and under conventional sterilization techniques which would greatly impair or destroy living tissues.
Over the past 20 years a number of trileaflet valves has been constructed. Various materials and combinations of materials such as epoxies, silicones, Teflon, Dacron, and polyester polyurethanes have been used. For example, the patent to Sako et al, U.S. Pat. No. 3,940,802 discloses a non-toxic compound for use in heart valves, comprising 100 parts by weight PVC and 50-100 parts by weight of polyurethane. Generally these valves have been clinically unsuccessful for the following reasons: Sinicone-Dacron polyester valves of a trileaflet configuration require the cusps to be made thick, relative to normal tissue valves, to give them strength and shape. Unreinforced silicone rubber is too weak to be used alone. Unfortunately, the Dacron reinforcement prevents efficient opening and closing of the valve and significantly reduces the flexural fatigue resistance of the silicone. Fracture of the fibers results in premature failure. Poor performance in the form of regurgitation of blood is a common problem with such stiff valves.
Uncoated fabric valves of Dacron or Teflon have proven unsatisfactory because of the loss of function due to heavy fiberous tissue ingrowth which impairs opening and closing. Teflon has been shown to rapidly fragment due to fatigue failure.
Polyurethane tri-leaflet valves attempted in the past suffer from the following defects: (a) Leaflets detach from their mounting frames when assembled from individual leaflets. (b) Leaflets flex-fatigue at the mounting frame when the frame is rigid, concentrating the flexural strain pattern at a point along the rigid-flexible junction. (c) The thin leaflet construction, when unreinforced, has a propensity to tear along its commissure line; reinforced materials are too stiff to function (d) Previous polyurethane leaflet valves were constructed of a hydrolytically unstable polyester polyurethane which rapidly degrades in the blood stream causing premature failure. (e) Thrombi generated on the surface were common occurrences with earlier designs using polyester polyurethanes. (f) Use of dissimilar materials can induce adverse reactions in designs where polyurethane leaflets are attached to an epoxy frame. In such cases, residual amines from the epoxy can migrate into the polyurethane causing degradation of physical properties and adverse surface characteristics may develop which can stimulate an inflamatory, toxic, or otherwise incompatible condition.
The patents to Hancock, U.S. Pat. No. 3,755,823; Parsonnet, U.S. Pat. No. 3,744,062, and Kischer, U.S. Pat. No. 3,548,417 are examples of prior art heart valves which suffer from one or more of the defects noted above.